A novel technique for fluorapatite synthesis and the thermodynamic mixing behavior of F-OH apatite crystalline solutions

Successful synthesis of fluorapatite has been achieved through ion-exchange between NIST hydroxlyapatite SRM 2910a and optical-grade fluorite. Additional intermediate F-OH apatite compositions were made through ion-exchange between the newly synthesized fluorapatite and the original hydroxylapatite. Based on solution calorimetric data collected on seven fluorapatite-hydroxlyapatite crystalline solutions at 50 °C in 20.0 wt% HCl under isoperibilic conditions, fluorine-rich series members display ideal thermodynamic behavior, whereas hydroxyl-rich compositions show negative enthalpies of F-OH mixing. Unit-cell volumes for the series are linear with composition. Relative to enthalpy and volume, therefore, there are no energy barriers to complete solid solution between the F and OH end-members.